The state of the art engine technology may allow a diesel engine to emit as low as 0.6 gm/mile particulates. However, with more stringent particulate emission requirements to come into effect in 1985 (such as at a level of 0.20 gm/mile) the technology cannot meet such lower level of particulate emissions without some form of particulate trap. The most important materials used to date by the prior art for the trap material have included rigid and fibrous ceramic material (see U.S. Pat. No. 4,276,071) and wire mesh, each material having its own characteristic mode of trapping. Some of these materials have been coated with catalysts to facilitate incineration, but the placement of the coating as a layer throughout the filter does not lower incineration temperature effectively and produces unwanted sulphates.
The particulates emitted and trapped throughout the life of a vehicle cannot be stored since the amount can be typically 20 ft.sup.3 for each 100,000 miles. As the particulates build up, exhaust system restriction is increased. Thus a means is required to remove the trapped material periodically. The most effective method found to date is thermal oxidation of the carbonaceous particles which incinerate at about 1200.degree. F. (600.degree. C.).
Normal diesel engine exhaust temperatures rarely reach 1100.degree. F. (600.degree. C.) during normal driving. Therefore, an auxiliary temperature elevating means is necessary to carry out thermal oxidation. The types of thermal oxidation means have generally fallen into the following categories: a fuel fed burner (U.S. Pat. No. 4,167,852 and Japanese Pat. No. 55-19934), an electric heater (see U.S. Pat. Nos. 4,270,936; 4,276,066; 4,319,896), or detuning techniques (which may be combined with the above) for raising the temperature of the exhaust gas temperature at selected times (see U.S. Pat. Nos. 4,211,075; 3,499,269). These techniques have been used to burn the collected particles in the presence of excess oxygen.
In all cases where regeneration means have been used by the prior art, the operation of the system has utilized the trap material in line during the regeneration cycle with no interruption of exhaust flow through the filter material (see U.S. Pat. Nos. 4,257,225 and 4,167,852), or has totally bypassed about the trap material by sending the exhaust gases around the trap to remain untreated during the regeneration of the trap material (see Japanese Pat. No. 55-19934, 1908).
With respect to the mode of regeneration where the trap material is in line, it has proven to be disadvantageous because of the excessive amount of energy required to raise the temperature of the total exhaust flow to the desired temperature for regeneration, and because the total trap is not used during normal operation. With respect to the bypass configuration of the prior art, it has proven to be disadvantageous because (a) no trapping takes place during the regeneration cycle, (b) the total trap material being not used during the normal trapping mode, and (c) additional exhaust silencing means may be required during regeneration.
Therefore, what is needed is a filtration system which has an operative mode of regeneration that (1) allows use of the entire filter material during trapping and regeneration, (2) reduces the energy required to regenerate by restricting the energy input to that which is necessary to ignite the collected particles and thereafter allowing the exothermic reaction to propogate and continue without further energy input, and (3) allows the use of simpler controls which operate independent of the engine operation.